It has been a common practice in the field of video processing to increase the apparent sharpness of an image by means of linear or nonlinear enhancement. Linear type enhancement techniques achieve sharper edges, or transitions, in a video signal simply by increasing, by diverse means (adding a second derivative of the signal for example) the amplitude of the high frequencies of the video spectrum. These techniques are simple and effective, but often create undesirable artifacts around edges, such as ringing.
Nonlinear enhancement is a more effective approach and attempts to reduce the rise time of an edge, also referred to as a transition, of an object by adding a signal derived from components of a main path video via nonlinear means in such a way as to increase the apparent resolution of the image without introducing significant ringing artifacts. The spectrum of the resulting video transitions is in effect widened, while a linear enhancement does not widen the spectrum, but increases the amplitude of its high-end frequencies, without extending the spectrum towards higher frequencies.
Several issued patents illustrate the state of the art in nonlinear enhancement. U.S. Pat. No. 4,030,121 by Faroudja, for example, discloses a system for improving the sharpness of the television picture by shortening transition time of single transitions occurring in video signals. The original input video signal is amplified and differentiated twice into first and second differentiated waveforms. The first differentiated waveform is rectified and then multiplied by a function of the second differentiated waveform. The waveform produced by the multiplication is then added to a delayed input video signal to provide an output signal having shorter transition times.
Conventional nonlinear enhancement techniques operate quite well in practice and expand in practice the signal spectrum by giving the illusion of a bandwidth that has been increased by the equivalent of roughly a 50% or a ratio of 1.5 to 2. The result is a picture having apparent increased sharpness with a reasonably acceptable level of artifacts. Such enhancement processes can be applied in the vertical as well as horizontal domains.
However, most if not all video signals include successive transitions of opposite signs, one following the previous one in a close interval of time, which is referred to as a double transition. An example of a double transition is a pulse of bright video on a dark background. As conventional enhancement are optimized for single transitions, and do not perform well on double transitions, a need has arisen for a transition enhancement process where single transitions, as well as pulses or double transitions, are enhanced in a manner that simulates the appearance of a video signal of wider spectrum.